Domain state capture using Libvirt
In order to aid application developers to choose which
operations best suit their needs, this page compares the
different means for capturing state related to a domain managed
by libvirt.
The information here is primarily geared towards capturing the
state of an active domain. Capturing the state of an inactive
domain essentially amounts to copying the contents of guest
disks, followed by a fresh boot of the same domain configuration
with disks restored back to that saved state.
One of the features made possible with virtual machines is live
migration -- transferring all state related to the guest from
one host to another with minimal interruption to the guest's
activity. In this case, state includes domain memory (including
register and device contents), and domain storage (whether the
guest's view of the disks are backed by local storage on the
host, or by the hypervisor accessing shared storage over a
network). A clever observer will then note that if all state is
available for live migration, then there is nothing stopping a
user from saving some or all of that state at a given point of
time in order to be able to later rewind guest execution back to
the state it previously had. The astute reader will also realize
that state capture at any level requires that the data must be
stored and managed by some mechanism. This processing might fit
in a single file, or more likely require a chain of related
files, and may require synchronization with third-party tools
built around managing the amount of data resulting from
capturing the state of multiple guests that each use multiple
disks.
There are several libvirt APIs associated with capturing the
state of a guest, which can later be used to rewind that guest
to the conditions it was in earlier. The following is a list of
trade-offs and differences between the various facets that
affect capturing domain state for active domains:
- Duration
- Capturing state can be a lengthy process, so while the
captured state ideally represents an atomic point in time
corresponding to something the guest was actually executing,
capturing state tends to focus on minimizing guest downtime
while performing the rest of the state capture in parallel
with guest execution. Some interfaces require up-front
preparation (the state captured is not complete until the API
ends, which may be some time after the command was first
started), while other interfaces track the state when the
command was first issued, regardless of the time spent in
capturing the rest of the state. Also, time spent in state
capture may be longer than the time required for live
migration, when state must be duplicated rather than shared.
- Amount of state
- For an online guest, there is a choice between capturing the
guest's memory (all that is needed during live migration when
the storage is already shared between source and destination),
the guest's disk state (all that is needed if there are no
pending guest I/O transactions that would be lost without the
corresponding memory state), or both together. Reverting to
partial state may still be viable, but typically, booting from
captured disk state without corresponding memory is comparable
to rebooting a machine that had power cut before I/O could be
flushed. Guests may need to use proper journaling methods to
avoid problems when booting from partial state.
- Quiescing of data
- Even if a guest has no pending I/O, capturing disk state may
catch the guest at a time when the contents of the disk are
inconsistent. Cooperating with the guest to perform data
quiescing is an optional step to ensure that captured disk
state is fully consistent without requiring additional memory
state, rather than just crash-consistent. But guest
cooperation may also have time constraints, where the guest
can rightfully panic if there is too much downtime while I/O
is frozen.
- Quantity of files
- When capturing state, some approaches store all state within
the same file (internal), while others expand a chain of
related files that must be used together (external), for more
files that a management application must track.
- Impact to guest definition
- Capturing state may require temporary changes to the guest
definition, such as associating new files into the domain
definition. While state capture should never impact the
running guest, a change to the domain's active XML may have
impact on other host operations being performed on the domain.
- Third-party integration
- When capturing state, there are tradeoffs to how much of the
process must be done directly by the hypervisor, and how much
can be off-loaded to third-party software. Since capturing
state is not instantaneous, it is essential that any
third-party integration see consistent data even if the
running guest continues to modify that data after the point in
time of the capture.
- Full vs. incremental
- When periodically repeating the action of state capture, it
is useful to minimize the amount of state that must be
captured by exploiting the relation to a previous capture,
such as focusing only on the portions of the disk that the
guest has modified in the meantime. Some approaches are able
to take advantage of checkpoints to provide an incremental
backup, while others are only capable of a full backup even if
that means re-capturing unchanged portions of the disk.
- Local vs. remote
- Domains that completely use remote storage may only need
some mechanism to keep track of guest memory state while using
external means to manage storage. Still, hypervisor and guest
cooperation to ensure points in time when no I/O is in flight
across the network can be important for properly capturing
disk state.
- Network latency
- Whether it's domain storage or saving domain state into
remote storage, network latency has an impact on snapshot
data. Having dedicated network capacity, bandwidth, or quality
of service levels may play a role, as well as planning for how
much of the backup process needs to be local.
An example of the various facets in action is migration of a
running guest. In order for the guest to be able to resume on
the destination at the same place it left off at the source, the
hypervisor has to get to a point where execution on the source
is stopped, the last remaining changes occurring since the
migration started are then transferred, and the guest is started
on the target. The management software thus must keep track of
the starting point and any changes since the starting
point. These last changes are often referred to as dirty page
tracking or dirty disk block bitmaps. At some point in time
during the migration, the management software must freeze the
source guest, transfer the dirty data, and then start the guest
on the target. This period of time must be minimal. To minimize
overall migration time, one is advised to use a dedicated
network connection with a high quality of service. Alternatively
saving the current state of the running guest can just be a
point in time type operation which doesn't require updating the
"last vestiges" of state prior to writing out the saved state
file. The state file is the point in time of whatever is current
and may contain incomplete data which if used to restart the
guest could cause confusion or problems because some operation
wasn't completed depending upon where in time the operation was
commenced.
With those definitions, the following libvirt APIs related to
state capture have these properties:
virDomainManagedSave- This API saves guest memory, with libvirt managing all of
the saved state, then stops the guest. While stopped, the
disks can be copied by a third party. However, since any
subsequent restart of the guest by libvirt API will restore
the memory state (which typically only works if the disk state
is unchanged in the meantime), and since it is not possible to
get at the memory state that libvirt is managing, this is not
viable as a means for rolling back to earlier saved states,
but is rather more suited to situations such as suspending a
guest prior to rebooting the host in order to resume the guest
when the host is back up. This API also has a drawback of
potentially long guest downtime, and therefore does not lend
itself well to live backups.
virDomainSave- This API is similar to virDomainManagedSave(), but moves the
burden on managing the stored memory state to the user. As
such, the user can now couple saved state with copies of the
disks to perform a revert to an arbitrary earlier saved state.
However, changing who manages the memory state does not change
the drawback of potentially long guest downtime when capturing
state.
virDomainSnapshotCreateXML- This API wraps several approaches for capturing guest state,
with a general premise of creating a snapshot (where the
current guest resources are frozen in time and a new wrapper
layer is opened for tracking subsequent guest changes). It
can operate on both offline and running guests, can choose
whether to capture the state of memory, disk, or both when
used on a running guest, and can choose between internal and
external storage for captured state. However, it is geared
towards post-event captures (when capturing both memory and
disk state, the disk state is not captured until all memory
state has been collected first). Using QEMU as the
hypervisor, internal snapshots currently have lengthy downtime
that is incompatible with freezing guest I/O, but external
snapshots are quick when memory contents are not also saved.
Since creating an external snapshot changes which disk image
resource is in use by the guest, this API can be coupled
with
virDomainBlockCommit()
to restore things back to the guest using its original disk
image, where a third-party tool can read the backing file
prior to the live commit. See also
the XML details used with
this command.
virDomainFSFreeze, virDomainFSThaw- This pair of APIs does not directly capture guest state, but
can be used to coordinate with a trusted live guest that state
capture is about to happen, and therefore guest I/O should be
quiesced so that the state capture is fully consistent, rather
than merely crash consistent. Some APIs are able to
automatically perform a freeze and thaw via a flags parameter,
rather than having to make separate calls to these
functions. Also, note that freezing guest I/O is only possible
with trusted guests running a guest agent, and that some
guests place maximum time limits on how long I/O can be
frozen.
virDomainCheckpointCreateXML- This API does not actually capture guest state, rather it
makes it possible to track which portions of guest disks have
changed between a checkpoint and the current live execution of
the guest. However, while it is possible use this API to
create checkpoints in isolation, it is more typical to create
a checkpoint as a side-effect of starting a new incremental
backup with
virDomainBackupBegin() or at the
creation of an external snapshot
with virDomainSnapshotCreateXML2(), since a
second incremental backup is most useful when using the
checkpoint created during the first. See also
the XML details used with
this command.
virDomainBackupBegin, virDomainBackupEnd- This API wraps approaches for capturing the state of disks
of a running guest, but does not track accompanying guest
memory state. The capture is consistent to the start of the
operation, where the captured state is stored independently
from the disk image in use with the guest and where it can be
easily integrated with a third-party for capturing the disk
state. Since the backup operation is stored externally from
the guest resources, there is no need to commit data back in
at the completion of the operation. When coupled with
checkpoints, this can be used to capture incremental backups
instead of full.
The following two sequences both accomplish the task of
capturing the disk state of a running guest, then wrapping
things up so that the guest is still running with the same file
as its disk image as before the sequence of operations began.
The difference between the two sequences boils down to the
impact of an unexpected interruption made at any point in the
middle of the sequence: with such an interruption, the first
example leaves the guest tied to a temporary wrapper file rather
than the original disk, and requires manual clean up of the
domain definition; while the second example has no impact to the
domain definition.
1. Backup via temporary snapshot
virDomainFSFreeze()
virDomainSnapshotCreateXML(VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY)
virDomainFSThaw()
third-party copy the backing file to backup storage # most time spent here
virDomainBlockCommit(VIR_DOMAIN_BLOCK_COMMIT_ACTIVE) per disk
wait for commit ready event per disk
virDomainBlockJobAbort() per disk
2. Direct backup
virDomainFSFreeze()
virDomainBackupBegin()
virDomainFSThaw()
wait for push mode event, or pull data over NBD # most time spent here
virDomainBackupEnd()